As a light diffusion sheet that is a constituent of a backlight unit used as a light source for various liquid crystal display devices such as a liquid crystal television, a light diffusion sheet formed from a resin composition prepared by adding a light diffusing agent to a matrix resin such as an acrylic resin or a polycarbonate resin is used.
For liquid crystal display devices such as liquid crystal televisions whose size has been increasing to 15 to 39 inches in recent years, a direct backlight system has been becoming mainstream along with an increase in area.
A light diffusion sheet made of acrylic resin has high moisture absorbability and low heat resistance and is therefore susceptible to the influence of an increase in size. Hence, when the light diffusion sheet made of acrylic resin is applied to this application, the warpage of the light diffusion sheet is liable to become great due to a change in use environment conditions. Accordingly, there occurs a serious problem that the light diffusion sheet makes contact with a liquid crystal board.
Meanwhile, a light diffusion sheet made of polycarbonate resin has excellent moisture absorbability, heat resistance, mechanical properties and weather resistance and high light transmittance. Thus, it is suitably applied to the application.
A light diffusing agent which is generally used for a polycarbonate resin is organic particles having a cross-lined structure. More specifically, cross-linked acryl particles and cross-linked silicon particles are suitably used from the viewpoint of a difference in refractive index between the particles and the polycarbonate resin.
One of the most important characteristics of light diffusion sheets used in a liquid crystal backlight application is brightness. Brightness is lightness which we sense visually and is considered as a correlation between the intensity of light and color (whiteness). Thus, brightness is lowered by a decrease in the amount of transmitted light caused by light absorption within the light diffusion sheet. Further, brightness is also lowered by yellowing of the light diffusion sheet. The polycarbonate rein is inferior to an acrylic resin in terms of brightness. Hence, an improvement in the brightness of the polycarbonate resin is desired.
As is generally known, the polycarbonate resin is liable to be yellowed by being subjected to a heat history in its production process (such as a reaction, powderization, drying or pelletization). Further, due to a light diffusing agent which is added to exhibit light diffusibility, the light diffusing agent itself undergoes yellowing at the time of molding or induces yellowing of the matrix resin.
A number of methods for improving the heat stability of a polycarbonate resin composition which contains a light diffusing agent are disclosed. For example, Patent Publication 1 describes that adding a phosphine compound and a hindered phenol based antioxidant to a polycarbonate resin prevents yellowing of the polycarbonate resin even when an organic light diffusing agent is used.
Patent Publication 2 describes use of an organic phosphite compound or hindered phenol as a stabilizer for a polycarbonate resin containing a light diffusing agent.
Further, Patent Publication 3 describes that a composition obtained by adding a phosphonate compound, trimethyl phosphate and hindered phenol to a polycarbonate resin containing a light diffusing agent undergoes inhibited discoloration caused by heat or moisture.
Further, Patent Publication 4 describes that a composition obtained by adding a phosphonate compound, trialkyl phosphate, pentaerythritol diphosphite and hindered phenol to a polycarbonate resin containing a light diffusing agent undergoes inhibited discoloration caused by heat.
However, the above polycarbonate resin compositions described in Patent Publications 1 to 4 are basically arts achieved solely from the viewpoint of prevention of discoloration of polycarbonate resin. Accordingly, although they have an effect of preventing discoloration, addition of various stabilizers and additives results in an increase in light absorbing species, so that they cause a decrease in the amount of transmitted light when used as a light diffusion sheet.
As for a method for improving brightness, light absorption in the light diffusion sheet must be suppressed, and discoloration such as yellowing must be inhibited as described above. That is, a light diffusion sheet for a direct backlight which has its brightness further improved by prevention of discoloration and suppression of light absorption is demanded.
(Patent Publication 1) JP-A 07-138464 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)
(Patent Publication 2) JP-A 07-149995
(Patent Publication 3) JP-A 2001-214049
(Patent Publication 4) JP-A 2001-323149